because every year samsung will release new One UI update with new android version, this is the major OS update every year
and the main cause of this light saber on the screen is because of extreme heat on the screen when the screen is on, and the heat burn the LED inside the screen and cause the green line
and most users have the habit of charging their phone when updating their phone because scared update halfway then no battery then phone will brick, and even samsung own OS will block you from updating your phone OS if the battery percentage is below 50%
hence users will charge their phone when updating the OS, charging phone will produce heat from the battery, and then update OS is a heavy task and hence will use alot of CPU/memory etc, this will create heat also, hence now the phone have 2 heat sources from charging and also the OS update
and what happen when u update your phone OS? the screen will always be turned on to show the progress of the update, so now the screen will take the extreme heat from the battery charging and OS update and thus burn the LED inside the screen, if lucky burn 1 line only , if unlucky then burn multiple lines
and so ngam new OS updates is always 1 year after the phone is released, thats why many ppl kena after 1 year